In a known production process, the plastic base material is forced under pressure to a pair of nozzle orifices provided in a mould block, via a network of supply ducts which are accommodated in a manifold in which they are heated by means of heating elements which are provided for this purpose in order to maintain the temperature of the supplied plastic material. From that point, the plastic material is injected in a mould, in which the abovementioned semi-finished products are produced by means of a series of successive operations, including metering and injecting the plastic material into the mould, pressing it down by means of the mould, cooling it and releasing it from the latter. During the injection phase, the liquid plastic material is injected into the mould between the core and cavity thereof. This also applies to end products such as a packaging container or closure of the lid type.
In known injection-moulding devices, the nozzle orifices are clamped between a so-called hot runner plate, in which the abovementioned network of supply ducts is accommodated and kept at the required temperature by means of a suitable heating means, and a cover plate. The free ends of the abovementioned nozzle orifices protrude therefrom so that the supplied plastic materials can be transferred to the mould.
The hot runner plate contains the manifold and the supply ducts which are heated therein and pushes against and closely adjoins the nozzle orifices. The operating temperature of the manifold and of the nozzle orifices is usually between 150° and 350° C., and in particular is approximately 300° C.
On the opposite side of the supply block, a clamping plate is fitted which covers the hot runner plate and in which the actuation of the abovementioned nozzle orifices takes place with effect at the location of their respective gates at their free end. The moulding on is controlled by means of reciprocating needles which block or clear said gates as a result of suitable actuation of the needles. In case an incident occurs at one of the various nozzle orifices of a multiple mould, the production of the mould has to be halted completely. This may be due to wear of an element in the flow path, soiling or narrowing of the flow ducts. In that case, the entire arrangement has to be dismantled, which is usually carried out from the rear side of the injection-moulding block. However, this creates a considerable problem as dismantling the injection-moulding device comprises the following steps. The clamping plate is unscrewed and removed, the manifold with all the flow ducts is detached and removed, the problematic nozzle orifice itself is eventually removed and replaced and subsequently everything has to be fitted back in reverse order. Thus, this replacement procedure for a single damaged nozzle orifice is relatively laborious, which is very disadvantageous. It is all the more disadvantageous since such devices comprise a large number of nozzle orifices.
In addition, the entire replacement procedure during this fitting has been found to carry a great risk of damage to the components, such as heating elements and temperature sensors which have been incorporated therein. In order to remedy this problem, these components are replaced as a preventive measure, but this in turn is associated with excessively high costs, which results in another problem.
However, the abovementioned problems of soiling and narrowing generally occur in the supply ducts which are not intended for the flow of a primary plastic base material, as these are neither dimensioned nor designed for this purpose. The supply ducts which are referred to here and which are the most problematic are however usually intended for more specific materials which may vary according to the application. Precisely because of this variability of application-specific secondary materials, it is virtually impossible to construct such flow ducts to exactly fit the materials which are to flow through them as these vary widely. Such circumstances may occur in the so-called multilayer technology which is used to produce multi-layered structures. These essentially consist of a primary base material which incorporates a secondary material in the form of a secondary layer contained in a primary base layer. However, this may also occur with a monolayer, such as PET/PET or PET, recycled PET or other materials. In such cases, a nozzle orifice which can be removed from the front has been in use for a long time.
With known multilayer systems, it is now impossible to no longer actuate the heating elements in order to switch off one or even more nozzle orifices during production, since this could cause cooling down of the system resulting in shrinkage, due to which the clamping of the nozzle orifices in the device, which are completely or partly made of metal, would no longer be optimum due to the metal composition, and neither would the connection between the manifold and the nozzle orifice. After all, all this would result in a leak at the location of the hot runner plate. This amounts to a leak of liquid plastic material at high temperature and pressure inside the device which would thus cause the cavities which have thus been created between said plates to be filled. The fatal consequence thereof would then be a total malfunction of the injection mould resulting in an undesired production shut-down. This situation has to be prevented at all costs, as the production process has to continue.
Thus, putting just one nozzle orifice of a multiple injection mould out of action is absolutely forbidden due to the construction of the latter. After all, since all nozzle orifices are directly connected to a communicating network of supply ducts through which the material stream is forced, one nozzle orifice directly affects the other in a mutual interaction. The immediate result thereof is that the entire system has to work in its entirety in order to be able to ensure that the semi-finished products which are to be produced are of good quality, in particular multilayer preforms or plastic containers.